The present invention relates to an apparatus and method for plasma etching. More particularly, it relates to an apparatus and method for plasma etching wherein a target film is etched by a plasma generated with a high-frequency induction field produced by a spiral coil disposed in opposing relationship with a sample stage provided in a chamber.
With the increasing miniaturization of a semiconductor integrated circuit element in recent years, exposing light with a shorter wavelength has been used in a lithographic step. At present, the use of a KrF excimer laser (with a wavelength of 248 nm) or an ArF excimer laser (with a wavelength of 193 nm) is becoming prevalent.
As the wavelength of exposing light becomes shorter, the reflectivity of light reflected from a substrate after exposing a resist film becomes higher so that the reflected light from the substrate is more likely to cause variations in the size of a resist pattern. To prevent the reflected light from being incident on the resist film, there has recently been used a process wherein an organic bottom anti-reflective coating (hereinafter referred to as ARC) is formed under the resist film. The ARC process is primarily used in the manufacturing of a semiconductor element in a high-performance device with design rules whereby a gate width is 0.25 .mu.m or less.
In the ARC process, it is necessary to etch the ARC after a resist pattern is formed by a conventional lithographic technique. Of a variety of plasma etching apparatus used to etch the ARC, an apparatus for inductively coupled plasma (ICP) etching with a spiral coil is used frequently.
As examples of the inductively coupled apparatus for plasma etching, an apparatus for inductively coupled plasma etching having a planar coil (see U.S. Pat. No. 4,948,458), an apparatus for inductively coupled plasma etching with a dome-shaped coil (see U.S. Pat. No. 5,614,055), and the like are known.
Referring to FIG. 10, a conventional apparatus for inductively coupled plasma etching having a planar single spiral coil will be described.
As shown in FIG. 10, a sample stage 2 as a lower electrode to which high-frequency power is applied is disposed in the lower portion of a grounded chamber I having an inner wall covered with an insulator such as ceramic, alumina, or quartz. A semiconductor substrate 3 as a sample to be etched is placed on the sample stage 2. The chamber 1 is provided with gas inlet ports (not shown) for introducing etching gas into the chamber 1 via a mass flow controller and with a gas outlet port 5 connected to a turbo pump for setting pressure in the chamber 1 to the order of 0.1 Pa to 10 Pa.
A single spiral coil 4 of inductively-coupled type is disposed atop the chamber 1 externally thereof in opposing relationship with the sample stage 2. The single spiral coil 4 has one end connected to a high-frequency power source via a matching circuit (not shown) and the other end connected to a wall of the chamber 1 and thereby grounded. The arrangement allows the single spiral coil 4 to generate a high-frequency induction field so that etching gas introduced into the chamber 1 is changed into a plasma. The etching gas changed into the plasma is guided by high-frequency power applied to the sample stage 2 toward the target film on the semiconductor substrate 3 held by the sample stage 2 so as to etch the target film.
When the present inventors performed an etching process with respect to an ARC as the target film by using a plurality of inductively coupled apparatus for plasma etching each having the planar single spiral coil 4 mentioned above, the problem occurred that the inplane uniformity of the etching rate varied with the different apparatus for plasma etching, though they were of the same model.
The inplane uniformity of the etching rate is defined as the degree of variations in etching rate across the surface of the target film and expressed as 3.sigma./.mu..times.100 (%), where .sigma. is the standard deviation of a data value and .mu. is the mean value of the data value. When variations in data value exhibit a normal distribution, 3.sigma. represents a deviation including 99.74% of the data value. The following equation (1) shows 3.sigma. and .mu. specifically. ##EQU1##
Conditions for the plasma etching process when the inplane uniformity of the etching rate was measured by using the conventional apparatus for plasma etching are as shown in Table 1.
In Table 1, ICP denotes high-frequency power applied to the single spiral coil 4 and RF denotes high-frequency power applied to the sample stage 2.
TABLE 1 N.sub.2 /O.sub.2 30/30 (sccm) ICP/RF 350/50 (W) PRESSURE 8 (mTorr) TEMPERATURE OF 10 (.degree. C.) SAMPLE STAGE
The models of apparatus for inductively coupled plasma etching and the inplane uniformities of the respective etching rates are as shown in Table 2. As shown in FIG. 18(a), etching was performed with respect to the ARC 11 as a target film formed on the semiconductor substrate 10.
TABLE 2 APPARATUS MODEL UNIFORMITY APPARATUS A .+-.4.5% APPARATUS B .+-.2.1% APPARATUS C .+-.5.6% APPARATUS D .+-.5.1% APPARATUS E .+-.3.3% APPARATUS F .+-.6.8% APPARATUS G .+-.2.6%
As will be understood from Table 2, the inplane uniformities of the etching rates for the ARC 11 were .+-.4.5% for the apparatus A, .+-.2.1% for the apparatus B, .+-.5.6% for the apparatus C, .+-.5.1% for the apparatus D, .+-.3.3% for the apparatus E, .+-.6.8% for the apparatus F, and .+-.2.6% for the apparatus G and not constant.
The etching rate which is inferior in inplane uniformity causes variations in the actual amount of etching across the surface of the target film. If the actual amount of etching varies across the surface of the target film, an adverse effect is produced such as variations in the characteristics of a FET in the case of forming the gate electrode of the FET by etching.